Cells of Dictyostelium discoideum are single amoeboid phagocytes. During the vegetative stage cells are chemotactic toward folic acid and during the aggregative stage toward extracellular cAMP. This cell is widely used as a model for amoeboid chemotaxis. From a number of studies it is clear that actin polymerization is stimulated by chemoattractant and that this is temporally correlated with pseudopod extension toward the source of chemoattractant. The general goal of this proposal is to determine how chemotactic stimulation is coupled to actin polymerization. We will investigate this by developing in vitro assays for ligand stimulated actin polymerization that is physiologically relevant to chemotaxis. We will then use these assays to investigate the molecular mechanism of coupling of actin polymerization to stimulation with chemoattractant. This investigation will include experiments using drugs and antibodies to modulate in vitro activity and eventual isolation of the ligand stimulated nucleation activity for actin polymerization.